The present invention relates to an electrophotographic device such as the laser printer and, more particularly, it relates to an electrophotographic device provided with a photosensitive body which has multi-photosensitive layers.
Generally, with respect to electrophotographics, a copy is obtained by uniformly charging a photosensitive body and then image-exposing it to form an electrostatic latent image. The electrostatic latent image is developed by charged particles (or developer) and the developed image is transferred onto material such as a sheet of paper. Then the transferred image is fixed on the transferring material by heating. In the case of this type electrophotographic device, the stability of the photosensitive body, which is used repeatedly, is important.
In this application, the primary component of stability is the stability of photoconductivity. The stability of images depends particularly upon how stable the electrostatic properties (e.g., charge potential and residual potential after irradiation) are when the photosensitive body is used continuously or with intervals.
The photosensitive body is usually subject to uniform exposure before it is charged to make the image stable. This procedure provides preparatory fatigue to the photosensitive body and eliminates any charge left when it is used repeatedly. This uniform exposure is performed with a device called either pre-exposure lamp, erasing lamp or pre-fatigue lamp. When the properties of the photosensitive body are stabilized by this uniform exposure, particular consideration must be paid particularly to those photosensitive bodies which are easily fatigued.
A manner of stabilizing the fatigue of an arsenic/selenium photosensitive body, for example, is disclosed in the Japanese Patent Disclosure No. Sho-53/148444. This document teaches that the initial variation of charge potential when the photosensitive body is used repeatedly may be held small by radiating the photosensitive body with light having a specific wavelength (a specific color), for the purpose, in particular, of strongly pre-fatiguing the photosensitive body prior to charging. A first green lamp and a second red lamp are provided to meet this purpose. In the copying mode, both the first and second lamps are turned on in the beginning and only the first lamp is kept on thereafter. The properties of the photosensitive body are stabilized by controlling the first and second lamps in this manner. In this case, the first green light is used for erasing the residual charges on the photosensitive body while the second red light is used just before the formation of the copy. The red light includes a wavelength of 620 nm by which the photosensitive body tends to be fatigued. Thus the first green light eliminates charge while the second red light provides pre-fatigue.
The stabilization of charge potential is also enhanced by gradually reducing the strengths of these uniform exposure lamps from the beginning of copying cycle.
The above-described manner of stabilizing the electrostatic properties of the photosensitive body has not reached such level yet that it can meet any change of environmental conditions such as temperature and humidity. It is suitable, however, in practical applications.
A selenium or silicon type photosensitive body is thought desirable which is sensitive to near infrared rays in the vicinity of 800 nm which is the wavelength of semiconductor lasers. An example of the selenium type is a Se/SeTe/Se/Al-based sensitive body and a Si-H-C/Si-H-Ge/Si-H-B/Al-based one is representative of the silicon type. Each has a multi-layer construction in which each layer is different from one another in spectral sensitivity. The reason why the photosensitive body has this multi-layer construction is, for example, that selenium/tellurium alloy is excellent in its sensitivity relative to the near infrared rays but abnormally quick in the dark decay of its charge. In order to cover the abnormal quickness of its dark decay, therefore, a layer of amorphous selenium (Se) or a layer made by adding a little amount of tellurium, antimony or arsenic is coated on it. Also in the case of amorphous silicon photosensitive body, its construction is often of multi-layer type for the same reason.
It was reported by A. R. Melnyk and others in "A Layered Se-Te Photoreceptor For A GeA As Laser Printer" at the "First International Congress On Advance In Non-Impact Printing Technologies" held by SPSE in June, 1981 that property deterioration of those photosensitive bodies which are sensitive to long wavelengths could be prevented by this multilayer construction. A method of manufacturing photosensitive bodies having superposed layers of Se-Te and Se-Te-Sb on a conductive support is disclosed in the Japanese Patent Disclosure No. Sho-56/151941.
Further, the amorphous silicon photosensitive body whose sensitivity relative to long wavelength has been increased by germanium is described in detail in the Japanese Patent Disclosure No. Sho-57/78183, for example.
Photosensitive bodies which had fundamentally the same construction have been manufactured on a trial basis. As the result, it has been found that their sensitivity relative to long wavelength can be enhanced, but that their electrostatic properties become extremely more unsatisfactory as compared with the conventional photosensitive bodies because of increase of persistent residual potential during continuous use and because of large reduction of charge acceptance when heated to a high temperature. These drawbacks are difficult to solve to meet practical purposes, and the electrophotographic devices using the photosensitive bodies of this type have not been practiced yet or have been provided as samples which can be used only under limited conditions.